1. Field of the Invention
The present invention refers to a fluorination catalyst based on an amorphous Cr (III) compound and on a compound of another metal of the 2a, 3a and 4b groups of the periodic system, preferably Mg or Ca, supported on AlF.sub.3 or fluorinated alumina, having an atomic ratio Cr/other metal higher than or equal to 1.
2. Description of Related Art
Such catalyst results particularly suitable in the fluorination in gaseous phase at atmospheric pressure of halogenated hydrocarbons with HF, in particular in the fluorination of CF.sub.3 CH.sub.2 Cl to CF.sub.3 CH.sub.2 F, perchloroethylene (PCE) to CF.sub.3 CHCl.sub.2, CF.sub.3 CHClF, CF.sub.3 CHF.sub.2 (HCFC 123, HCFC 124, HFC 125 respectively), with high selectivity, efficiency and life-time.
The fluorination catalysts based on Cr (III) compounds, preferably supported on alumina, fluorinated alumina, aluminum trifluoride or carbon, are known in the art and industrially used for preparing HCFC and HFC not harmful for the ozone layer.
However such chromium catalysts have the drawback to cause also secondary reactions which lead to the formation of very undesirable by-products.
For instance, in case of fluorination of CF.sub.3 CH.sub.2 Cl (HCFC-133a) to CF.sub.3 CH.sub.2 F (HFC-134a), it is also obtained the formation of CF.sub.2 .dbd.CHCl (HCFC-1122) which is not only toxic but is also difficult to be separated from 134a, requiring to this purpose further expensive purification processes such as those described in U.S. Pat. No. 5,475,168.
In the case of preparation of HFC-125, undesirable amounts of CFC-115 are obtained, said CFC being very difficult to separate from HFC-125 (see U.S. Pat. No. 5,087,329).
Moreover said Cr catalysts tend to deactivate since, during the use, there are deposited on their surfaces carbonaceous and/or oligomeric residues deriving from cracking and/or oligomerization of the organic compounds put to react or formed during the reaction, in particular of the unsaturated ones.
In order to be able to reactivate the catalyst it was therefore necessary to stop at intervals the manufacturing process to proceed to the catalyst regeneration, generally by oxidation with air at high temperatures of the carbonaceous deposits.
To overcome such drawback it is known in the prior art to add to the reaction mixture small amounts of oxygen (see e.g.
WO 90/08755). This addition of oxygen, while effective in sustaining the catalytic activity, has nevertheless two major drawbacks: the formation of halogenated epoxides, dangerously unstable, and the reaction of oxygen with the HCl which is formed in the main reaction, to give molecular chlorine which in turn reacts with the HFC/HCFCs forming CFCs that contaminate the desired products. EQU 2 RCHX.sub.2 +2 HCl+O.fwdarw.2 RCX.sub.2 Cl+H.sub.2 O EQU (X=F, Cl; R=halogenated alkyl)
The CFCs, as widely known, cannot be discharged into the atmosphere because of their impact on the ozone layer, and must thus be properly disposed of. This requires addition units in the industrial process.
A further disadvantage of the known catalysts is their capacity to promote the disproportionation of the organic compounds. For example, from monohydrogenated halocarbons (120 series) one obtains di- and zero hydrogenated halocarbons (130 series and 110 series or CFCs). This is particularly disadvantageous when the desired product be HFC-125, since among the 110 series there is formed CFC-115, which is extremly difficult to separate from 125 (see U.S. Pat. No. 5,087,329).
Other chromium and magnesium containing catalyst are known in the art: see e.g. U.S. Pat. No. 5,559,069 which refers to a catalyst containing Cr and Mg fluorides without any reference to the preparation of the 120 series. Moreover when the fluorination of an unsaturated starting material is carried out, the catalytic activity is very low and the selectivity is of no practical industrial interest.
In EP 417,680 it is described the preparation of 134a from 133a and HF in gaseous phase in the presence of an unsupported mixed catalyst based on Cr (III) and Mg, wherein the atomic ratio Cr/Mg is equal to or lower than 0.66.
Such catalyst is prepared according to the process described in U.S. Pat. No. 4,547,483, comprising the reaction of 1 mole of a Cr(III) soluble salt with at least 1.5 moles of Mg oxide or hydroxide in aqueous solution, with formation of a paste containing Cr hydroxide and Mg salt, which, after drying, is treated with HF at temperatures from 20.degree. to 500.degree. C.
It is stated that said coprecipitation method is unsuitable to the preparation of supported catalysts, and that Mg has the sole purpose to act as a binder, to confer to the catalyst the desired mechanical properties.
Said catalyst when employed under high pressures (5-15 bar) results particularly effective in reducing the amount of olef inic by-products and in particular that of CF.sub.2 .dbd.CHCl(HCFC-1122) while if employed at atmospheric pressure it does not result effective in such reduction.
The use of high pressures implies however a greater cost of the fluorination industrial plants as regards the materials and the structural parts of the plant which must be not only resistant to high mechanical/chemical stresses but must meet more severe rule requirements meant to avoid possible risks of leakage of HF and of the reaction products.
EP 657,408 describes a fluorination process in gaseous phase with HF of halogenated aliphatic hydrocarbons, in particular of HCFC-133a in the presence of a mixed crystalline catalyst based on Cr oxide or oxides and at least another metal catalytically active selected from Ni, Co, Mn, Mg, Fe, Zn and V.
However such crystalline catalyst quickly deactivates in operation: for instance, a crystalline catalyst based on Mg CrO.sub.4 and Cr.sub.2 O.sub.3 deactivates after only 24 hour of operation.